1. Field of the Invention
The present invention generally relates to a camera module for a motor vehicle, comprising a lens objective, a lens holder holding said lens objective, and a back plate connected to said lens holder and holding an image sensor in or close to an image plane of the lens objective. The invention also relates to a method of pre-focusing a lens objective in a lens holder.
2. Description of Related Art
Automotive camera modules are exposed to temperatures varying over a large range, and are required to have excellent imaging performance over the whole temperature range occurring in practice. In many fixed-focus automotive camera modules, a major challenge is to retain the focus over the temperature envelope T. For low f/# lenses, the distance between the image and the sensor must be controlled at a scale of micrometers. However, the lens holder expands with increasing temperature, and therefore the distance between the lens objective and the image sensor increases along the optical axis with increasing temperature, with the result that the sensor moves out of the image plane of the lens objective and the image quality deteriorates.
DE 10 2008 047 277 A1 discloses a camera module where a compensating tube is provided between a lens carrier and a lens barrel. The compensating body may have an inner thread into which the lens barrel may be screwed.
DE 102 61 973 A1 discloses a camera module where the lens is indirectly held by a housing tube via a frusto-conical lens holding element which has a significantly smaller thermal expansion coefficient than the housing tube.
The camera modules of DE 10 2008 047 277 A1 and DE 102 61 973 A1 provide for a compensation of the thermal expansion of the lens holder through a compensating element which, however, constitutes an additional part and makes the mounting of the camera module more laborious.